Metal nanoparticles have recognized importance in chemistry, physics and biology because of their unique optical, electrical and photo-thermal properties. Such metallic nanoparticles have potential applications in laboratory settings such as analytical chemistry and have been used as probes in mass spectroscopy, as well as in the colorimetric detection for proteins and DNA molecules. Metal nanoparticles have also been used for therapeutic applications and drug delivery. However, their use in the general public has garnered a lot of attention and investigation. For many years silver (Ag) has been known to possess antibacterial properties and this characteristic has been exploited in a wide variety of applications, such as catheters, protheses, textiles, water treatment, etc.
The mechanism for the antimicrobial property of silver is only partially understood. It has been hypothesized that the positively charged Ag is able to interact with the negatively charged bacteria cell wall, inhibiting membrane permeability (see Ratte, H. T., Bioaccumulation and toxicity of silver compounds: a review, Environ. Toxicol. Chem. 1999, 18, 89-108), inactivating necessary enzymes by interaction with the thiol groups of the proteins (see Gupta, A.; Maynes, M.; Silver, S., Effects of halides on plasmid-mediated silver resistance in Escherichia coli, Appl. Environ. Microbiol. 1998, 64, 5042-5045; see also Matsumura, Y.; Yoshikata, K.; Kunisaki, S.- i.; Tsuchido, T., Mode of bactericidal action of silver zeolite and its comparison with that of silver nitrate, Appl. Environ. Microbiol. 2003, 69, 4278-4281), leading to cell death.
Silver nanoparticles also have the same intrinsic property. The toxicity of silver to microbes is largely due to silver ions (Ag+), which are very toxic to microbes. The use of silver nanoparticles is of high interest because of the slower more controlled release of the Ag+. The molecular mechanism for the antimicrobial effects of silver nanoparticles has been hypothesized to be due to the metallic silver)(Ag° being oxidized to silver ions upon exposure to water or other oxidizing agents (James, G. V., Water Treatment; A Survey of Current Methods of Purifying Domestic Supplies and of Treating Industrial Effluents and Domestic Sewage. 4th ed. 1971; p 311 pp.).
The smaller the particle the more surface area is exposed to water forming more silver ions which can deactivate the proteins necessary for bacteria, viruses, and fungi to survive. The slower release of silver cations from silver nanoparticles can avoid the constant delivery of an excess amount of silver to the area compared with other Ag+ based chemicals. Using silver nanoparticles, the metallic silver is not as susceptible to deactivation by the chloride molecules compared with the Ag+ (Dunn, K.; Edwards-Jones, V. The role of Acticoat with nanocrystalline silver in the management of burns; Wythenshawe Hospital Burns Unit, Manchester, UK: England: United Kingdom, 2004; pp S1-9).
Theoretically, nanoparticles have a greater surface area relative to their mass. This increased ratio means greater antimicrobial activity and a more controlled release of the toxic Ag+ ions. See Dunn, K.; Edwards-Jones, V. The role of Acticoat with nanocrystalline silver in the management of burns; Wythenshawe Hospital Burns Unit, Manchester, UK: England: United Kingdom, 2004; pp S1-9. See Baker, C.; Pradhan, A.; Pakstis, L.; Pochan, D. J.; Shah, S. I., Synthesis and antibacterial properties of silver Nanoparticles. J. Nanosci. Nanotechnol. 2005, 5, 244-249. See Morones, J. R.; Elechiguerra, J. L.; Camacho, A.; Holt, K.; Kouri, J. B.; Ramirez, J. T.; Yacaman, M. J., The bactericidal effect of silver Nanoparticles. Nanotechnology 2005, 16, 2346-2353.
Various methods have been reported over the last two decades for the synthesis of silver nanoparticles which involve the reduction of metal salts with a chemical reducing agent, such as sodium citrate, sodium borohydride, or other organic compounds. See Plyuto, Y.; Berquier, J.- M.; Jacquiod, C.; Ricolleau, C., Ag Nanoparticles synthesised in template-structured mesoporous silica films on a glass substrate. Chemical Communications (Cambridge) 1999, 1653-1654. See Rivas, L.; Sanchez-Cortes, S.; Garcia-Ramos, J. V.; Morcillo, G., Growth of silver colloidal particles obtained by citrate reduction to increase the Raman enhancement factor. Langmuir 2001, 17, 574-577. See Tan, Y.; Jiang, L.; Li, Y.; Zhu, D., One Dimensional Aggregates of Silver Nanoparticles Induced by the Stabilizer 2-Mercaptobenzimidazole. J. Phys. Chem. B 2002, 106, 3131-3138. See Zhang, Z.; Patel, R. C.; Kothari, R.; Johnson, C. P.; Friberg, S. E.; Aikens, P. A., Stable silver clusters and Nanoparticles prepared in polyacrylate and inverse micellar solutions. J. Phys. Chem. B 2000, 104, 1176-1182.
Unfortunately, using such reducing agents introduce chemicals that are biologically incompatible or environmentally toxic. Thus the conventional synthesis of silver nanoparticles incorporates contaminants that could pose problems in biomedical applications.
A need therefore exists for biologically friendly reducing agents for synthesizing silver nanoparticles in a manner that prevents contamination with toxic chemicals.